Re: [PATCH net-next] tcp: add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb()

On 2021/9/2 9:13, Eric Dumazet wrote:

On Wed, Sep 1, 2021 at 5:47 PM Yunsheng Lin [off-list ref] wrote:

quoted

On 2021/9/1 18:39, Yunsheng Lin wrote:

quoted

Since tcp_tx_skb_cache is disabled by default in:
commit 0b7d7f6b2208 ("tcp: add tcp_tx_skb_cache sysctl")

Add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb() to
avoid possible branch-misses.

Signed-off-by: Yunsheng Lin <redacted>
---
Also, the sk->sk_tx_skb_cache may be both changed by allocation
and freeing side, I assume there may be some implicit protection
here too, such as the NAPI protection for rx?

Hi, Eric
   Is there any implicit protection for sk->sk_tx_skb_cache?
As my understanding, sk_stream_alloc_skb() seems to be protected
by lock_sock(), and the sk_wmem_free_skb() seems to be mostly
happening in NAPI polling for TCP(when ack packet is received)
without lock_sock(), so it seems there is no protection here?

Please look again.
This is protected by socket lock of course.
Otherwise sk_mem_uncharge() would be very broken, sk->sk_forward_alloc
is not an atomic field.

Thanks for clarifying.
I have been looking for a point to implement the socket'pp_alloc_cache for
page pool, and sk_wmem_free_skb() seems like the place to avoid the
scalablity problem of ptr_ring in page pool.

The protection for sk_wmem_free_skb() is in tcp_v4_rcv(), right?
https://elixir.bootlin.com/linux/latest/source/net/ipv4/tcp_ipv4.c#L2081

TCP stack has no direct relation  with NAPI.
It can run over loopback interface, no NAPI there.
.